Electrosensitive recording process and sheets



April 30, 1963 F. A. HAMM ET AL 3,087,869

ELECTROSENSITIVE RECORDING PROCESS AND SHEETS Filed March 3l, 1960 United States Patent "Office 3,087,869 Patented Apr. 30, 1963 3,087,869 ELECTRSENSITIVE RECORDING PROCESS AND SHEETS Frank A. Hamm, Stillwater, and Richard L. Weiher,

St. Paul, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Mar. 31, 1960, Ser. No. 18,903 15 Claims. (Cl. 204-2) This invention relates to an electrosensitive recording sheet. In one aspect lthis invention relates to an electrosensitive recording paper for facsimile signals and the like.

Trends in modern technology have been placing increasing demands on fast, -convenient and inexpensive techniques for printing line copy or continuous tone images and also for recording coded information. For example, recording information such `as speech and pictorial or scientific data from high speed aircraft or satellites in lorbit is assuming considerable significance. Also high speed printing of the output of electronic computers is still seriously limited by the fact that the nal recording stage is slow in comparison t the computation rate of the machine.

Various methods of recording by means of electric signals and Various types of recording media have been known, but in general prior art methods and, recording blanks employed therein have been subject to certain disadvantages. For example, it has been known to irnpregnate paper with electrically conductive solutions, or electrolytes, said solutions being capable of decomposition by signal impulses so that a record of the signals is made. It is also old to cause an electric arc to burn holes in a paper recording blank, the holes thus burned representing telegraphic signals for comprising the outlines of the subject matter desired to be recorded. Such recordings, however, are only suitable for relatively coarse work where no fine gradations of tonal values are necessary, such as in the reception of telegraphic signals in general and the recording of various types of curves in connection with process instrumentation. However, in the transmission of matter, particularly of pictures and the like, the above methods have suffered from serious disadvantages, such as diffusion of the image area.

One of the most widely used types of electrosensitive paper consists of a carbon impregnated conductive paper bearing, on one surface, a metallic paint layer and, on the other surface, a powdery light gray coating. During the recording process the light gray coating is at least partially displaced or removed by the passage of current from the stylus through the paper. Such sheets are difficult to handle because of the readily smeared, readily pressure marked surface. The current ow tends not only to alter the character of the light gray coating but also to disrupt the bond-ing or adhesion of the light gray masking coating to the adjacent carbon impregnated paper and to provide thereby a readily removable image.

Attempts have been made in the art to produce electrosensitive sheets which do not rely on electrolytes or on the physical disruption of the sheet. Certain materials have been employed which decompose under the influence of heat generated by an electric current or which alter their form in an allotropic manner. applications, as in various instruments measuring process variables, such sheets are subject to extremes of temperature, pressure and the like. A satisfactory heat stable, electrosensitive sheet which is capable of producing a high contrast, permanent, well defined smudge resistant image has heretofore been an elusive objective.

Facsimile as used herein is not limited to the conventional facsimile image recording process but also en- However, in certain' Compasses other electronic data reproduction processes, such as telegraphy, weather mapping, intraofce and interotiice business copying, recording of scientific information, electronic computer output recording, etc.

It is therefore an object of this invention to provide such an electrosensitive paper with the above-mentioned and other advantages.

It is an object of this invention to provide an electrosensitive copysheet capable `of reproducing smudge resistant, highly stable images with good definition and high contrast.

It is another object of this invention to provide an electrosensitive sheet which is resistant to pressure marking, mechanically strong, and essentially unaffected by moisture, light, elevated temperatures and salt solutions.

An additional object of this invention is to produce an electrosensitive facsimile sheet capable of reproducing an image with substantially no d-usting, flaking or smoking and which can be reused upon erasure of the image.

Another object of this' invention is to provide an image bearing sheet which can be further developed by electrolytic means and a process for such development.

FIGURE l is a cross-sectional View of an electrosenstitive sheet according to this invention.

FIGURE 2 is a schematic diagram of a facsimile recording system embodying an electrosensitive copysheet 2 according to this invention on a rotating roll 3 grounded through shaft 4. Moving stylus 1 makes contact with the surface of copysheet 2.

FIGURE 3 is a schematic diagram of a rotating drum facsimile recorder using an electrical contact type pickup stylus.

TheV electrosensitive sheets of this invention comprise a continuous electrically conductive layer land a contiguous layer comprising finely divided indium oxide in a matrix of a binder having a lower electrical conductivity than said indium oxide. For the production of a superior facsimile image, the conductivity through the indium oxide containing layer should be essentially uniform over its entire area. Variations in conductivity, such as are caused by a lack of uniformity in layer thickness and by photoconductivity of the indium oxide containing layer,

tend t-o produce non-uniform image density and erratic development. Sensitivity `to light, i.e. photoconductivity, is therefore not necessary for purposes of the instant invention and is particularly undesirable for the aforementioned reasons.

The electrically conductive layer may be any conductive material, including metal foil, paper or fabric impregnated with conductive particles, such as aluminum flakes, vapor deposited ymetal films, conductive glass, conductive plastics, etc. Flexible sheets are generally used, particularly when the electrosensitive sheet is wrapped a rotatable roll or drum during the receipt `of electrical facsimile signals and reproduction of the image. Although the nondisplaceable or nondisruptable nature of the contiguous layer or electrosensitive topcoating permits the use of a carbon impregnated matrix, such as carbon impregnated paper, as the electrically conductive layer, this is not preferred because of other generally less desirable properties of carbon impregnated matrices, for example, relatively low tear strength, particularly in a high humidity environment, etc.

The electrosensitive layer or topcoating contains a binder having a low electrical conductivity in which is uniformly distributed or dispersed particulate indium oxide, which can be reduced to a lower oxidative state, primarily free indium metal, 'by the passage of electrical current, eg. by means of a stylus, in the presence of small amounts of an hydrogen ion containing electrolyte, such as ordinary tap water. For purposes of this invention,

such hydrogen ion containing electrolytes shall be referred to as aqueous electrolytes. This current passage results in the electrolytic reduction of the particulate indium oxide, and the free indium metal thereby produced in those areas provides a visible reproduction of the electrical impulses or currents. Although the mechanism is not fully understood, it is believed that the electrolytic reaction produces nascent hydrogen which, in turn, reduces the indium oxide to free indium metal. Y

In bonding the indium oxide to the electrically conductive base layer or support sheet various flexible, filmforming binders having a lower electrical conductivity than indium oxide can be employed. Organic polymeric binders are particularly satisfactory in many applications because of their water resistance, including materials such as polystyrene, chlorinated rubber, rubber hydrochloride, a 30 weight percent solution of 30:70 mole ratio copolymer of butadiene and styrene in toluene, etc. However, a water soluble or hydrophilic binder containing water and having a lower electrical conductivity than indium oxide, such as gelatin, can also be used as will be later described.

Since it is often desirable to have a whiter appearing recording sheet, certain dyes which tend to absorb light in the yellow portion of the visible spectrum may be incorporated into or on the surface of the indium oxide containing layer. Some of these dyes are commonly referred to as optical bleaches, and the stilbene dyes, such as Blankophor R, Blankophor B, etc., are particularly representative of such materials. The normally off-white or pale yellow color of the indium oxide can also be improved somewhat by reducing the particle size or by adding minor amounts of iinely divided white pigments, such as titanium dioxide.

The coating containing indium oxide is prepared by first mixing together the finely divided indium oxide powder and the binder material, preferably in a solvent for the' binder. Various ratios of indium oxide and resinous binder may be employed, depending on the properties most desired in the final coating, although optimum results are usually obtained with indium oxide pigment to binder ratios from about 2:1 to about 10:1, the ratio selected being determined in par-t by the particular binder used. Thus, particularly when a binder such as a 30:70 mole ratio butadiene-styrene copolymer is employed, the weight ratio of indium oxide to binder preferably ranges from about 5:1 to about 7:1.

Before coating the indium oxide-binder mixture onto the electrically conduc-tive backing it is necessary to provide an intimate suspension of the indium oxide in a solution of binder in a volatile solvent for the binder. The amount of volatile solvent, eg. toluene, ethyl acetate, etc., is controlled to prevent gelling before -the coating operation and to provide a viscous spreadable suspension which is smooth and free of lumps. Dry coating thickness of the lm usually varies `from about 0.1 to 20 microns, with a preferable thickness of from about 5 to microns. The thinner coatings have the advantage of a lower electrical impedance and thus allow lower voltages to be used in the developing process.

The indium oxide is electrolytically reduced in the above sheet construction, using the preferred water resistant or water insoluble binders, when the electrically conducting substrate or backing is connected as the cathode and an anodic stylus is brought into contact with the indium oxide coating in the presence of an aqueous electrolyte. When the binder material is water soluble and can serve as a source of hydrogen ions, e.g. gelatin, no additional electrolyte is required. In such `an embodiment, the stylus may be connected as either the anode or the cathode. The indium oxide reduction occurs at the indium oxide layer-conductive backing interface when the stylus is anodic and a-t the exposed surface adjacent the stylus when the stylus is cathodic. In both cases, the dark colored indium metal produces a visible image, although the image resoiution is improved by effecting the indium oxide reduction at the exposed or outer surface.

Depending on the speed at which the stylus traverses the surface of the indium oxide coated sheet, as well as the conductivity of the coating, the voltage applied across the construction may range from as little as about 1.5 volts to much higher values. The electrolytic reduction of the indium oxide is generally independent of the rate of electrical charge passed per unit area, and the optical density of the image areas increases with charge passed. The passage of an electrical charge as low as 150 millicouloumbs per square centimeter serves to eiciently reduce indium oxide to indium metal and to provide a diffuse reflected den-sity of 1.0. At the higher writing or recording speeds, higher voltages serve to provide the necessary charge per unit area at the proportionately lesser contact time. For example, with about l0 micron thickness of the indium oxide layer, a voltage of about 20 volts D C. is suicient to permit practical stylus speeds of the order of 64,000 inches per minute. Voltages below about 200 volts D.C., particularly below about volts D.C., are preferable, since the higher voltages tend to cause undesirable physical disruption of the indium oxide layer, especially at the slower writing speeds.

As mentioned earlier, the reduction of indium oxide to free indium is an electrolytic process requiring the presence of at least small amounts of an aqueous electrolyte or current conducting hydrogen ion containing medium, such as water. lReduction of the indium oxide will not occur in the complete absence of such an electrolyte. Techniques which require neither a fixing nor a washing step are utilized in the development of the image, c g. on conventional rotating drum facsimile recording apparatus. A ydamp sponge can be positioned forward of the moving stylus to provide the necessary electrolyte, or the indium oxide surface can be moistened prior to use in some other manner. To obviate the need for a wetted surface, an aqueous electrolyte can be incorporated onto the `surface of the sheet construction, as by providing a thin surface coating of gelatin which serves as the -required electrolyte or by using gelatin and the like as the binder material, 'as already discussed.

Image reproduction has been made both of line copy and of continuous tone with standard facsimile stylus writing techniques, with resulting images that display excellent contrast and resolution. The chemical composition of the stylus is not critical, since no chemical reaction involving the Istylus is required. However, the stylus material should not only be electrically conductive but also should not be decomposed by the electrolytic reaction. Because the resolution in the image area varies inversely with the size of the stylus contact area or point, it is also desirable lto provide a `stylus which is resistant to abrasion and wear, such materials as molybdenum and tungsten being outstanding in this respect.

A still further feature of the indium oxide facsimile or record-ing sheets of this invention resides in the erasibility of the indium metal image. Erasure can be effected by treating the image-containing sheet surface with acid, such as nitric acid, which dissolves or oxidizes the indium metal without affecting the unreduc'ed indium oxide. Other oxidizing techniques may also be employed. Facsimile sheets so erased can be reused for recording further information.

The dense, high contrast black images formed in the process of this invention are 4highly stable and are unaffected by boiling `for 15 minutes in both water at 100 C. and a `sodiu-m chloride solution at C. Even exposure to an air atmosphere at 76% relative humidity and 80 C. for hours produces little noticeable effect on the image. No observable change has been detected upon exposure to window light for 5 days and toartificial light for about 30 days.

The following examples are set forth for purposes of illustrating certain embodirnentsof the invention and are not to be construed as limiting.

Example 1 A 6:11 weight ratio of In2O3 powder and a 30:70 mole ratio copolymer `of butadiene and styrene (3() weight percent solution in toluene) were ballmilled lfor 24 hours and coated 5 mils wet on aluminum laminated paper. The resulting lm was allowed to dry for 24 hours. Using the rotating drum appara-tus depicted schematically in FIG- URE 3, the above sheet 2 was wrapped around a 2l inch circumference steel r-oll 3. A Mylar sheet `4, containing a metallic-silver image 5 grounded to the steel drum, was wrapped around another section of the drum. Two copper contacts in fixed relationship to one another were mounted to provide contact with each of the sheets respectively, and the entire dual stylus assembly was movable at a fixed rate of speed in a direction parallel to the axis of rotation of the steel drum. A 20 volt D.C. source was provided between the two contacts or stylii. With the steel drum rotating at 100 r.p.rn. and the stylus assembly moving at :a speed of 0.025 inch per drum revolution, the indium oxide containing sheet was moistened slightly by means of a sponge containing a suitable electrolyte, such as an aqueous 1 molar (NH4)2SO4 solution. As the one stylus contacted the silver image, thus closing the electrical circuit, the indium oxide coating in contact with the other stylus was darkened, the oxide being reduced to indium metal. The silver image was thereby reproduced with excellent definition and contrast on the indium oxide surfaced sheet.

Example 2 A standard Western Union facsimile machine, using a photocell pickup, was substituted Ifor the rotating steel drum apparatus of Example l. With a 2() volt A.C. voltage on the printing stylus, a drum (3 inches circumference) speed of 90 r.p.m., and a print speed of 10U-120 lines per inch, an excellent reproduction I,of the image was produced on a water dampened indium oxide coated sheet similar to the sheet of Example 1.

Example 3 An indium oxide coated sheet, prepared as in Exam ple 1, was coated with gelatin by dissolving gelatin powder in tap water at about 501 C. and applying to the indium oxide layer. After the gelatin had hardened, an aluminum probe connected as anode was pressed into contact with the gelatin layer. The aluminum backing of the sheet was connected as the cathode, and a voltage of 6 volts D.C. was impressed across the sheet construction. A dense brownish black indium metal mark was readily formed at the gelatin-indium oxide layer interface.

Example 4 A mixture of indium oxide powder, gelatin powder and water was blended by hand and coated onto the metallized surface of aluminum laminated paper. After the coating had hardened, an aluminum probe was pressed into contact with this coating and a direct current voltage was applied across the construction. When the aluminum substrate served as cathode and the aluminum probe as anode, reduction of the indium oxide to free indium occurred at the substrate-coating interface. When the polarity was reversed, reduction was localized at the aluminum probe-coating interface, providing the more visible indication or marking.

Various other alterations and modifications of the present invention will be apparent to those skilled in the art without departing from the scope of this invention.

We claim:

l. An electrolytic process for the recording of facsimile and other electrical impulse information which comprises passing an electrical current through a sheet comprising a continuous electrically conductive backing having superimposed thereon a homogeneous coating of finely divided indium oxide and a film forming binder having lowerelectrical conductivity than said indium oxide, said indium oxide being in contact with an aqueous electrolyte, thereby electrolytically reducing the indium oxide powder to free indium.

2. The process of claim l in which the binder is a water-resistant, polymeric material having a lower electrical conductivity than the indium oxide and in which said electrically conductive backing is connected as cathode.

3. The process of claim l inwhich the binder is gelatin.

4. An electrolytic process for the recording of facsimile and other electrical impulse information which comprises contacting with a stylus the water moistened surface of a light insensitive sheet comprising a continuous electrically conductive backing having superimposed thereon a homogeneous coating of finely divided indium oxide and a film forming binder having a lower electrical conductivity than said indium oxide, and passing suflicient electrical current through said sheet between said stylus and said electrically conductive backing to electrolytically reduce indium oxide adjacent said moistened surface to free indium.

5. The process of claim 4 in which said stylus is connected as anode and said electrically conductive backing is connected as the cathode.

6. An electrolytic process for the recording of facsimile and other electrical impulse information which comprises contacting with a stylus the water containing surface of a light insensitive sheet comprising a continuous electrically conductive backing having superimposed thereon a homogeneous coating of finely divided indium oxide powder and a water resistant polymeric binder having a lower electrical conductivity than said indium oxide, and passing sufficient current through said sheet between said stylus as anode and said electrically conductive backing as cathode to electrolytically reduce indium oxide adjacent said water containing surface to free indium.

7. An electrolytic process for the recording of facsimile and other electrical impulse information which comprises contacting with a cathodic stylus the moistened surface of a light insensitive sheet which comprises a continuous electrically conductive backing having superimposed thereon a homogeneous coating of finely divided indium oxide powder and a moisture-containing gelatinous binder, and passing sufficient current through said electrosensitive sheet between said stylus and said electrically conductive backing as anode to visibly reduce said indium oxide to free indium.

8. A light insensitive facsimile recording sheet which comprises a continuous electrically conductive backing having superimposed thereon a homogeneous coating of nonphotoconductive indium oxide powder and a binder having a lower electrical conductivity than said indium oxide.

9. The facsimile recording sheet of claim 7 in which the binder is a water-resistant polymeric material.

10. The facsimile recording sheet of claim 7 in which the binder is a moisture-containing gelatinous binder.

l1. A light insensitive facsimile recording sheet which comprises a continuous electrically conductive backing having superimposed thereon a homogeneous coating of a non-photoconductive indium oxide powder and a binder having a lower electrical conductivity than said indium oxide, said coating being of essentially uniform transverse electrical conductivity.

l2. A llight insensitive facsimile recording sheet which comprises a continuous electrically conductive backing having superimposed thereon a homogeneous coating of a non-photoconductive indium oxide powder and a -binder having a lower electrical conductivity than said indium oxide, said coating being of essentially uniform thickness.

13. A light insensitive facsimile recordingL sheet which comprises a continuous electrically conductive backing, an intermediate homogeneous coating of non-photoconductive indium oxide powder and a binder having a lower electrical conductivity than said indium oxide, and a top coating of gelatin.

14. The electrosensitive recordingr sheet of claim l1 in Which the Weight ratio of indium oxide 4to binder is between`22l and 10:1.

15. A light-insensitive, stable electrosensitive recording sheet which comprises a continuous electrically conductive backing having superimposed thereon a homogeneous coating of uniform thickness comprising a finely divided non-photoconductive indium oxide powder and a binder matrix having a lower electricalv conductivity than said indium oxide. Y Y

References Cited in the le of this patent UNITED STATES PATENTS Talmey Sept. 19, 1939 OTHER REFERENCES Westbrook: Trans Am. Electrochem. Soc., volume 57, pages 290-29l, 1930.

Handbook of Chemistry and Physics, 38th ed., 1956, page 52'8. 

1. AN ELECTROLYTIC PROCESS FOR THE RECORDING OF FACSIMILE AND OTHER ELECTRICAL IMPULSE INFORMATION WHICH COMPRISES PASSING AN ELECTRICAL CURRENT THROUGH A SHEET COMPRISING A CONTINUOUS ELECTRICALLY CONDUCTIVE BACKING HAVING SUPERIMPOSED THEREON A HOMAGENEOUS COATING OF FINELY DIVIDED INDIUM OXIDE AND A FILM FORMING BINDER HAVING LOWER ELECTRICAL CONDUCTIVELY THAN SAID INDIUM OXIDE, SAID INDIUM OXIDE BEING IN CONTACT WITH AN AQUEOUS ELECTROLYTE, THEREBY ELECTROLYTICALLY REDUCING THE INDIUM OXIDE POWDER TO FREE INDIUM. 